The present invention relates to a new and improved method of measuring a characteristic value essentially corresponding to a quantity of opened fibres which is being transported in an air stream to a spinning preparatory machine and controlling the supply of opened fibrous material into the air stream, and further pertains to an improved construction of apparatus for implementing the aforesaid method.
It is already known to the art to measure the volume or the weight, respectively, of pneumatically transported bodies, particularly opened fibre flocks, by means of optical scanning or measuring operations. From the electrical signals which are generated as the flocks individually pass through a light beam of the scanning or measuring device, and the duration of which signals corresponds to the duration of the interruptions of the light beam, it is possible to derive the fibre weight after suitable transformation of electrical pulses. This technique presupposes that the bodies or the fibre flocks, as the case may be, do not pass through the duct cross-section simultaneously but individually in succession, since e.g. two overlapping flocks interrupt the light beam in the same manner as one single flock of corresponding size. This method furthermore requires that the flocks are statistically evenly distributed in the transporting duct in order to obtain a value representative of the total through passing quantity of fibre flocks and which can be used for controlling the flock material supply. With this technique too high a transporting speed also can detrimentally influence the measurement, since then the duration of the interruptions become too brief and owing to the inertia of the scanning or measuring device no longer can be sufficiently precisely detected. These restrictive requirements thus only permit determination of the effective production, i.e. the quantity of fibrous material transported per time unit, with limited precision and which is insufficient for certain applications which occur in spinning preparatory plants where there is required the highest precision.
Furthermore, there is known to the art a random-laid web card on which there is produced a web or lap of uniform density. The fibrous material delivered by two clothing-covered rolls is taken-off the last clothing-covered roll by means of a tangential air stream and is transferred via a duct to a rotating perforated suction drum arranged immediately after the clothing-covered roll, the air flowing-off via the deposited web to the interior of the perforated drum. In the section of the duct located between the clothing-covered roll and the perforated drum, and in which duct section the web is already partially formed, there is arranged a pressure measuring device. Pressure variations caused by the varying web density are detected by this pressure measuring device and the speed of the supply rolls feeding the clothing-covered rolls with fibrous material is controlled as a function of the change in pressure, i.e. the web density. The measuring device in this case thus detects a change in the aerodynamic characteristics of the air stream which depends upon the penetration resistance of the air stream through the material already deposited on the perforated drum, i.e. upon the density of such material. This device is therefore associated with the drawback that it is not possible to determine the quantity of material which is being transported.
In British Pat. No. 1,206,610, there is taught a valve for a pneumatic unloading and loading device for pulverulent material (cement), by means of which there can be adjusted or set a desired air-material mixture as a function of the kinetic conditions prevailing in the pneumatic transport duct. The material infeed to the transporting air stream occurs in this case as a function of the pressure difference of the transporting air stream and thus the air throughput, this pressure difference prevailing at a throttle. The greater the air throughput that much more material is supplied and vice versa, so that with varying air throughput there is always maintained one and the same desired air-material-mixture. A pre-determined and constant production of material cannot be realized with this prior art equipment, since the quantity of material likewise alters during a change of the air throughput.
Now in French Pat. No. 1,199,574, it is known from the ironworks industry, to deliver pre-determined quantities of lime powder and oxygen for refining a metal melt, wherein there is adjusted a gas pressure corresponding to the desired quantity of lime and in order to maintain the desired powder-oxygen-mixture constant. Since a desired quantity of pulverulent lime can be obtained by adjusting the gas pressure, in order to realize a desired gas-material-mixture, it is not possible to achieve a predetermined and constant production of material, if, with a gas pressure which has been once set, the arriving material quantity changes.